I'd be nice if all cars got 50mpg like your Prius.
I still have a clunky '94 Pathfinder, best case 19-20mpg so for me, 4 gallons round trip from Santa Clara (if I lived there) at $4.00/gal = Caltrain is still better $$-wise and carbon-footprint-wise.
I just saw a show that had a car from Volvo called the "Recharge", a plug-in hybrid with a 60-mile range before the gas engine runs to recharge the batteries only. I'm going to get a car like that when it becomes available and affordable.